1. Field of the Invention
The present invention generally relates to an optical module used in the field of wavelength division multiplexing optical communication technology and in the field of spectrometric technology. One aspect of the invention relates to an optical path changing optical system in an optical module. Further, another aspect of the invention relates to a mechanism for holding optical parts constituting an optical module. 2. Description of Related Art
On the other hand, in the field of spectrometric technology, a diffraction grating is used widely for analyzing spectra of light in a spectrometric apparatus. Spectrometric analysis requires high efficiency in utilization of energy in a wide frequency band. A reflection-type diffraction grating is suitable for obtaining high diffraction efficiency in a wide frequency band. The reflection-type diffraction grating is used widely in a spectrometric apparatus because the reflection-type diffraction grating is good in the ratio of change in diffraction angle to the wavelength of light, that is, in wavelength-angle dispersion characteristic (e.g. see Tadao Tsuruta, “Applied Optics 1”, Baifukan Co., Ltd, 1990, p. 307).
Particularly in the field of environmental measurement or the like, there is an increasing demand for a small-size spectrometric apparatus which can be carried to a job site and which can perform measurement on the site. Several small-size sensors for measuring the optical spectra of a liquid flowing in a fine flow path or evaluating the property such as pH of the liquid by using the optical spectra of the liquid have been proposed and a small-size spectrometric apparatus has been used in the field of environmental measurement.
Also in the small-size spectrometric apparatus used for these purposes, a reflection-type diffraction grating is used as a spectral distributing element in the same manner as in the large-size spectrometric apparatus according to the related art. Generally, a mirror or the like is used for contriving an optical system to achieve reduction in size of the spectral distributing function.
A diffraction grating is further used in a light demultiplexer in the field of wavelength-division multiplexing optical communication. When a reflection-type diffraction grating is used for aligning the optical axis of incident light and the optical axis of diffracted light on a line approximately, that is, when an optical system of so-called Littrow arrangement is formed, a small-size light demultiplexing module can be achieved (e.g. see International Patent Publication No. 99/46638 Pamphlet).
The reflection-type diffraction grating however has such a property that the positional relation between incident light and diffracted light varies sensitively with dependence on the mounting angle of the diffraction grating. For example, as shown in FIG. 16, incident light 402 containing wavelength components λ1, λ2 and λ3 is collimated to parallel light beams 404 by a lens 430 and then incident on the reflection-type diffraction grating 410. The light beams diffracted by the diffraction grating 410 outgo in different directions in accordance with the wavelengths. For example, the diffracted light beam component of the wavelength λ2 outputs in a direction in which the angle (diffraction angle) between the optical axis 407 of the diffracted light beam component and a perpendicular line 405 drawn from a substrate 412 of the diffraction grating is β2. The diffracted light is converged so as to be incident on an element 420-2 such as a photodetector of a diffracted light detection device 420.
When the angle (incident angle) α between the optical axis 403 of incident light and the perpendicular line 405 drawn from the diffraction grating substrate 412 changes on this occasion, the angle (angle γ2 for the diffracted light with the wavelength λ2 in the aforementioned example) between the optical axis 403 of incident light and the optical axis 407 of diffracted light also changes. It is therefore necessary to adjust the mounting angle of the diffraction grating strictly relative to incident light and adjust the mounting position of the diffracted light detection device strictly. There is a problem that it is difficult to assemble an optical module.
Also for this reason, in the optical module using the reflection-type diffraction grating, a complex mechanism for adjusting the positions of respective parts needs to be provided in a housing of the module or an adhesion/fixation space for holding an element needs to be reserved in advance. Accordingly, there is a problem that the size of an apparatus using the optical module is increased.
Turning to the field of opto-electronics, the development of technique has advanced for partially replacing the electric signal transmission/signal processing between devices or in a device with the optical signal transmission/processing. Such a technique needs an inter-conversion device between the optical signal and the electric signal as well as signal transmitting and processing device for transmitting and processing the optical signal and the electric signal respectively. Therefore, if optical elements and electronic elements taking charge of these functions are mixedly mounted on a board so that both optical signal and electric signal can be transmitted, processed and inter-converted on one and the same board, it is possible to enjoy a lot of advantages such as improvement in efficiency of signal processing and reduction in device size. In a so-called opto-electronic hybrid circuit board where the optical elements and the electric elements are mixedly mounted, it is necessary to provide an optical system by which light propagating in a light pipe such as an optical fiber, an optical waveguide, etc., or propagating in a space can be received by a flat photodetecting element mounted in the board or light emitted from a flat light-emitting element mounted on the board can be taken out to a light pipe or a spatial optical path mainly in a portion of conversion between the optical signal and the electric signal.
It is preferable from the viewpoint of an easy layout and a small mounting space that the optical fiber, the optical waveguide and the spatial optical path are arranged so that the optical axes of these are in parallel to the board. On the other hand, the optical element for performing inter-conversion between the optical signal and the electric signal is mounted on the board. Therefore, in an optical system having such optical arrangement, a so-called flat optical element having incident and output surfaces parallel to the board surface is preferably used so that the optical path parallel to the board is bent by 90° so as to be connected perpendicularly to the flat optical element mounted on the board.
Although the angle for changing the optical path may be an acute angle or an obtuse angle other than 90°, this case brings lowering of efficiency and difficulty of adjusting because incidence/output on/from the optical element has an inclination angle. If light is intended to be incident/outgo perpendicularly on/from the optical element, complexity in device configuration and increase in device size are brought because the optical path of propagating light cannot be made in parallel with the board surface.
It is however unnecessary to adjust the optical path changing angle to 90° accurately. Even in the case where an angle several degrees far from 90° is selected as the design bending angle, the aforementioned problem does not become so actual if the mounting structure of each optical element is designed in accordance with the angle. The “90° optical path change” in the following description means roughly 90° optical path change including variation in such a width of several degrees.
The 90° optical path change can be achieved when reflection of light is used. Generally, it can be achieved by use of a mirror or prism (e.g. see Japanese Patent Publication No. 2004-85913A).
In the opto-electronic hybrid circuit board, a slight difference in optical path causes a large light loss because the beam diameter of light to be used, the core diameter of the light pipe to be coupled, the area of the photodetecting surface of the flat photodetecting element, and so on, are small. For this reason, a high degree of precision is required for the mounting angle, the angle of a reflection surface or surface accuracy in order to perform the 90° optical path change. There arises a problem that a complex assembling or producing process is required.